This invention relates to an apparatus for detecting pinholes in cans and in particular to an improvement of apparatus for detecting pinholes in cans by radiating light toward the cans.
Cans made of thin metal plates, such as tin, tinfree-steel and aluminum plate are broadly used as containers for hermetically filling with refreshments, pharmaceuticals, cosmetics, and other products for daily life. In these kinds of cans, pinholes or cracks (cracks are referred to as pinholes below) are caused from failure in the can manufacturing line and the mixture of nonmetals into the can materials. As a result, the problem of leakage out of the cans arises. Therefore it is necessary to detect pinholes in the can manufacturing line.
The air pressure method and the transit light method are known as conventional pinhole detection methods. The pinhole detection apparatus using the air pressure method detects changes in the internal pressure of the cans by a pressure detector or diaphram gauge which keeps the internal pressure high or low, and measures the internal pressure of cans of which the ends are sealed with rubber packing. This air pressure method is suitable to detect pinholes in the band seam (double seam) portion of the cans or the junction of the body and the bottom of the cans and leakage in the band seam (double seam) portion due to the failure of the winding-up seam. But defects as described below are found in this air pressure method. That is, the internal pressure variation resulting from pinholes and other reasons is generally very slow in the variation speed, therefore it requires a long detection time. For example, in case a can having 200 cc capacity has a pinhole of 50.mu. diameter, it requires a minute for certain detection. At this time, air pressure of 1 kg/cm.sup.2 is given as the initial pressure, and then pressure reduction of 50 g/cm.sup.2 must be detected. As will be apparent from this description, in the conventional air pressure method, a number of can detections require much labor and time.
Also, in the air pressure method, the opening end of the cans must be steadily pressed to the rubber packing to maintain high or low pressure within the cans. As a result, the rubber packing frequently blocks up pinholes in a processing flange of the can opening. It is very difficult for this method to detect pinholes in the adjacent opening end.
In addition, coatings or print inks cover the pinholes having diameters of 100.mu. or less in the cans in which the inside and the outside thereof are coated or printed for cosmetic or prevention of corrosion. This makes the pinholes detection difficult when the detection is made.
On the other hand, the transit light method detects pinholes by directing irradiation light to the outside of the cans and then by detecting light passing through pinholes into the interior of the cans. This method is preferably suitable to detect pinholes in cans having no band seam (double seam) portion. Namely, the transit light method is available to detect pinholes since the ratio of DI (Drawing and Ironing) cans manufactured by the drawing and ironing processing is increasing these days. In the DI cans the body and bottom are incorporated in one by the drawing. The conventional pinhole detection apparatus with the light detection method is known in U.S. Pat. No. 3,416,659, and a schematic representation thereof is shown in FIG. 1.
In FIG. 1, an opening end (10a) of the DI can (10) tightly contacts a light seal plate (14) of a dark box (12) with the DI can (10) standed upside down. The box (12) has a window (12a) for communicating with the interior of the can (10). A vacuum pump (not shown) reduces the internal pressure within the box (12) and the can (10) through an exhaust opening (16) provided with a side wall of the box (12). As a result, the opening end (10a) and the light seal plate (14) are tightly contacted with each other by the difference between the internal and the atmospheric pressures. The light seal plate (14) is of opaque rubber packing. An engagement of the opening end (10a) in the light seal plate seals light. Light sources (18) are above and around the can (10) which is exposed to both the direct light from the sources (18) and the reflected light from a cylindrical reflector (20) around the sources. A photo detector (22) provided within the box (12) detects light passing through pinholes from the light sources (18) and then converts its light into an electrical output (100) through a signal circuit (not shown). Thus, pinholes in the can are detected.
According to the conventional apparatus of FIG. 1, light from the light sources (18) through pinholes into the interior of the can (10) reaches the photo detector (22) directly or reaches it with light repeatedly scattered and reflected within the inside wall of the can (10). The photo detector (22) is capable of detecting pinholes anywhere on the can (10) by the optical integral action of the reflected, scattered light, and therefore possibly acquires the high sensitive pinhole detection action.
However, in this conventional apparatus, leaked light out of the seal portion between the opening end (10a) of the can (10) and the light seal plate (14) produces detection noises. To reduce the amount of this leaked light, the reduced internal pressure provided within the box (12) and the can (10) is made to tightly contact the opening (10a) with the light seal plate (14). As a result, wear of the light seal plate (14) becomes extreme. Generally, in the conventional apparatus, the can (10) is detected with the condition that the flange is formed in the opening end (10a), in other words, the end surface of the opening end (10a) is rounded. But even with this roundness the opening end (10a) results in extreme wear of the light seal plate (14) of the rubber packing. In general, a side use of two to three weeks and both sides use of about six weeks make frequent replacements of the parts, and make maintenance of the detection apparatus difficult. In addition, the use of the easily worn light seal plate (14) results in error detections.
FIG. 2 shows other samples with respect to the light seal plate (14) of the conventional rubber packing. These samples illustrate the structures for reducing the leaked light out of the tight contact portion between the opening end (10a) and light seal plate (14). FIG. 2A shows a dish-shaped concave portion provided on the tight contact surface with the opening end (10a), which makes it easy to receive the opening end (14). But, on the contrary, its structure does not fit nicely the opening end (10a) with the light seal plate (14), therefore both must be strongly pressed to each other. FIG. 2B shows a funnel-shaped light seal plate (14). In this other example, in the same way as FIG. 2A, unfitness of the opening end (10a) to the light seal plate (14) increases the amount of leaked light. Still further, FIG. 2C shows a -shaped sectional light seal plate (14) capable of tightly contacting the opening end (10a) with the light seal plate (14) with slight contact pressure. But, too much contact pressure and irregularity of axis position of the can to the light seal plate (14), produce uneven deformation of the light seal plate (14), and hence a large amount of leaked light occurs. Also it requires much time to adjust contact pressure or contact position. Further, the pressure or changes of the contact position must be managed.
Also, in the conventional apparatus, in the same way to the air pressure method, pinholes are covered with coatings or printing ink. Especially, detecting pinholes of diameters of 100.mu. or less become extremely difficult. Practically, the cans having pinholes covered with coatings or print inks keep good air-tightness for only a few days. In most cases, pinholes produce leakage after a week and remarkably increase the ratio of the defective cans. In particular, the defective cans having substances have a practical influence on common consumers. This is a very big problem.
As will be apparent from the above description, pinholes are preferably detected before the coating or printing operation. But as well known, in the DI processing operation, the drawing or ironing processed DI can is made a necking and flange processing in the opening end. The shape remains as it was before the rounded end surface processing in the can end is made. The trimmed end surface has a width such as about 0.17 mm, which is very thin. As a result, when the opening end (10a) tightly contacts the light seal plate (14), the light seal plate (14) is remarkably worn or is easily broken in a particular case. Therefore, it is impossible to detect pinholes before coating or printing.
This invention relates to improvement of the above conventional apparatus.
An object of this invention is to provide a can pinhole detection apparatus with very high detection accuracy which remarkably elongates maintenance and exchange term of the light seal plate or has no necessity of replacements of the light seal plate by improving the light seal plate.
An object of this invention is to provide a can pinhole detection apparatus for precisely detecting defective cans without attenuating the transit light into the interior of the can.
Another object of this invention is to provide a can pinhole detection apparatus in which the maintenance is made with ease and the light seal plate with wear resisting property has a long replacement life.
Another object of this invention is to provide a can pinhole detection apparatus having the light seal plate with a conical-shaped introduction, which makes it easy to insert an opening end of the can therein.
A further object of this invention is to provide a can pinhole detection apparatus in which a slight pressure is given to the can in order to easily close the opening end of the can.
A further object of this invention is to provide a can pinhole detection apparatus in which the light seal plate has a groove for accepting the open end of the can therein and is of harder material than that of the can.
A still further object of this invention is to provide a can pinhole detection apparatus in which the groove surface of the light seal plate is treated with specific materials to attenuate the reflection of light.
To achieve the above objects, this invention is characterized in that a light seal plate has a groove for accepting an open end of a can and is of very hard material in a pinhole detection apparatus. The apparatus comprises a dark box having a window through which an interior of the can communicates with the light seal plate shutting incidental light by tightly contacting the open end of the can therewith, a photo detector electrically detecting the incidental light within the dark box and light sources.
The above and other advantages of the invention will become more apparent in the following description and the accompanying drawings.